Alcohol-induced depletion of polyunsaturated lipids alters the physical and functional properties of biological membranes. Chronic alcohol exposure is known to cause resistance to ethanol-induced membrane disordering measured in vitro. Fluorescence studies in our laboratory confirm that this response recovers 2 to 3 days post-withdrawal in rat liver microsomes. Bulk membrane "fluidity" measurements with DPH detect no difference in basal membrane order between controls and liver microsomes from the withdrawal group. However, a localized membrane probe, TMA-DPH, detected a significantly higher basal membrane order in the 24 hours post-withdrawal group. Motions of fluorescent probes localized in specific areas of these bilayers suggest that alcohol-induced changes in lipid composition affect membrane organization in very distinct lipid domains. Furthermore, our laboratory has shown that alcohol depletes the long chain polyunsaturated fatty acids such as 20:4n6 and 22:6n3 from membranes. We hypothesize that there is a specialization in the localization and function of the 22:6-phospholipids in biological membranes. To test this hypothesis, the physical environment of 22:6n3 and other polyunsaturated phospholipids have been studied using fluorescence anisotropy and differential scanning calorimetry of membrane preparations. We have found that under defined conditions, bilayers containing 22:6n3 species become more highly packed than bilayers containing species with fewer double bonds. However, correlation times of the same labeled samples indicate that the local environment along the fatty acyl chains becomes increasingly disordered with increasing unsaturation.